Four morphologically distinct tayassuid species are present in the marine strata of the lower Chesapeake Group in Maryland and Virginia. The oldest of these, an unnamed species, occurs in bed 2 of the Calvert Formation and is the only terrestrial mammal yet known from this unit. In the upper Calvert and lower Choptank Formations three tayassuid species resemble closely in size and morphology taxa known from the middle and upper Miocene Olcott and Valentine Formations in Nebraska. “Cynorca proterva” and “Prosthennops” xiphidonticus are present in beds 10 and 14, respectively, of the Calvert Formation. “Prosthennops” niobrarensis occurs in bed 17 of the Choptank Formation. These taxa, considered together with the other known terrestrial mammals from the Chesapeake Group, indicate the following correlations with land mammal faunas from western North America: bed 2, ?late Arikareean or early Hemingfordian; bed 10, late Hemingfordian or early Barstovian; bed 14, early late Barstovian; bed 17, late late Barstovian. These correlations are largely concordant with micropaleontogical correlations for the Chesapeake Group, but the age indicated for bed 17 is younger than an age estimate based on diatoms.

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